Environmentally harmful species in exhaust gas emitted from an internal combustion engine, such as hydrocarbons (HC), carbon monoxide (CO), particulate matters (PM), and nitric oxides (NOx) are regulated species need to be removed therefrom. In lean combustion engines, due to the existence of large amount oxygen excess, passive means without extra dosing agents, such as that using a three-way catalyst, normally are not able to effectively remove the oxidative specie NOx, as that in most of spark-ignition engines. To reduce NOx in lean combustion engines, a variety of active means with reducing agents (reductants) being dosed in exhaust gas are developed. In these technologies, a dosing amount of reductant is injected into exhaust gas, and the result mixing gas flows into a SCR catalyst, where the reductant selectively reacts with NOx generating non-poisonous species, such as nitrogen, carbon dioxide, and water.
A variety of reductants, such as ammonia (NH3), HC, and hydrogen (H2) can be used in SCR systems. Among them, ammonia SCR is used most broadly due to high conversion efficiency and wide temperature window. Ammonia can be dosed directly. However, due to safety concerns and difficulties in handling pure ammonia, in ammonia SCR systems, normally ammonia is obtained from a urea solution through thermolysis and hydrolysis, and the urea solution in these applications is also called reductant. In mobile applications, typically a eutectic solution of urea, i.e. a 32.5% wt urea solution, is used.
In a SCR system, dosing accuracy significantly affects NOx control performance, especially when engine out NOx level is high. For example, if engine out NOx is 1000 ppm, with a NSR (Normalized Stoichiometric Ratio) of 1.0, 1000 ppm ammonia is needed. If an uncertainty of 5% exists in dosing control, then 50 ppm of ammonia could be over- or under-generated. Though in transient, SCR catalyst has certain capability storing ammonia, clamping the effects of under-dosing and over-dosing, in average, these effects may still cause issues. In the example above, if the SCR system is tuned for an average doser with zero storage usage, then an under-dosing doser may create 50 ppm NOx slip in average, which is almost 0.3 g/bhp·hr in normal operations. Compared to the US 2010 emission standard of 0.2 g/bhp·hr, it is 150% uncertainty. This calculation just includes the doser uncertainty. Other important factors, such as NOx sensor error, urea decomposition error, and exhaust flow rate error, also contribute to the overall control uncertainty.
Using SCR catalyst with large storage capability (e.g. a Cu-zeolite catalyst) together with an AMOX (AMmonia OXidation) catalyst desensitizes NOx control to NSR. Thereby uncertainties in dosing system and sensors can be compensated through over-dosing. However, relying on the storage capability of SCR catalyst and AMOX could cause aging issues, since both of the storage capability of SCR catalyst and the selectivity of AMOX are subject to aging effects. An aged AMOX tends to oxidize ammonia slip back to NOx, and an aged SCR catalyst has lower deNOx efficiency. Therefore, the aging of the catalysts may cause NOx slips with over-dosing, especially when temperature variation releases stored ammonia in SCR catalyst.
All these issues cause difficulties in applications in which high deNOx efficiency has to be maintained. For example, to reach the requirements of 0.2 g/bhp·hr (emission limit) to 0.4 g/bhp·hr (OBD limit) set by the US2010 and CARB2016 emission regulations, when an engine out NOx level is 4.0 g/bhp·hr, a deNOx efficiency of 95% is needed for normally operations and when the deNOx efficiency drops below 90%, a fault needs to be generated. The high efficiency requirements for SCR systems cause difficulties in controlling NOx level and detection failures due to effects of uncertainties in dosing system and sensors, resulting in high system and warranty costs.
To lower the deNOx efficiency requirement, engine out NOx concentration has to be limited to a low level with EGR (Exhaust Gas Recirculation) technology, and retarded fuel injection. However, too much EGR and fueling retard may deteriorate engine operating performance and fuel economy. Additionally, a trade-off between NOx emission and PM emission exists in engine control. Lowering engine out NOx normally causes increase in PM emission and fuel economy is further deteriorated due to that more energy needs to be consumed in regenerating the DPF which traps PM.
In an SCR system, under a given condition, deNOx efficiency is limited by the effective volume of its catalyst, which is determined by the uniformity of exhaust gas and reductant, and the catalyst volume, while most of SCR reactions and catalyst aging happen at the front end of the catalyst. To obtain higher deNOx efficiency with lower warranty costs, a multi-stage SCR system including more than one SCR catalysts and dosing apparatus can be used. In the multi-stage SCR system, exhaust gas with unreacted ammonia can be re-mixed with newly dosed reductant, and thereby a better uniformity and higher deNOx efficiency can be achieved. At the same time, the isolation between a front SCR device, in which most of aging happens, and a back SCR device with less aging decreases difficulties in detecting catalyst failures and lowers system warranty cost, since more likely only a smaller front SCR needs to be replaced in case of a catalyst failure.
However, a multi-stage SCR normally needs a complex structure, which may significantly increase system cost or cause other issues. For example, when air assisted dosers are used in a multi-stage SCR system, each SCR needs a dosing module and an independent control. The system complexity thus is significantly increased. If airless dosers are used, adding a new dosing branch creates issues in pressure control and dosing accuracy due to the effects of injector actions to reductant pressure. Also, in a purge process, when air enters from one pressure line to the pump in the airless doser, the pump pressure could drop significantly, causing issues in pulling reductant residue in the other pressure line, resulting in purging failures.
To solve the problems in multi-stage SCR systems so that high deNOx efficiency and easy failure detection can be obtained with simple structure and low warranty cost, a primary object of the present invention is to provide a multi-stage SCR system with a scalable dosing system in which a reductant delivery branch can be added with minimum effect to it structure and dosing performance.
A further objective of the present invention is to provide a multi-stage SCR system with more uniform exhaust flow, so that higher system deNOx efficiency can be obtained.
Another objective of the present invention is to provide a multi-stage SCR system in which no extra sensor is needed compared to a single-stage SCR system.
Yet another objective of the present invention is to provide a multi-stage SCR system in which deNOx efficiency in the rear portion of the overall SCR catalyst can be higher than that of the first stage SCR, so that the system is less sensitive to catalyst aging.